The present invention relates to an image display device and method, more particularly to a method of digitally processing an image signal to clarify lines, dots, and edges.
Images are displayed physically by a variety of devices, including the cathode-ray tube (CRT), liquid-crystal display (CRT), plasma display panel (PDP), light-emitting diode (LED) display, and electroluminescence (EL) panel. To display color images, these devices have separate light-emitting components for three primary colors, normally red, green, and blue.
In a CRT display, the separate colors are produced by a repeating pattern of red, green, and blue phosphor dots or stripes. FIG. 1 shows how a round white dot having a width of seven phosphor stripes, for example, is displayed. Electron beams illuminate red phosphors Rb, Rc, green phosphors Ga, Gb, Gc, and blue phosphors Ba, Bb in the spatial pattern shown. FIG. 2 maps the luminance distribution of this displayed dot in the horizontal direction. The distribution has separate luminance centroids R′, B′, G′ for the three primary colors, but all three centroids are disposed near the center of the dot, near phosphor Gb in this example.
The other types of display devices mentioned above are flat panel matrix display devices comprising two-dimensional arrays of picture elements (pixels). In a color matrix display, each pixel includes separate cells of the three primary colors. For example, FIG. 3 shows an LCD pixel comprising a red cell R1, a green cell G1, and a blue cell B1. Personal computers often have matrix-type displays of this type.
Although there is a trend toward increasing resolution in matrix-type displays, it is difficult to fabricate a display screen with extremely small pixels, especially when each pixel comprises three separate cells. Since there is also a trend toward the display of increasing amounts of information on the display screen by the use of small fonts, it is not unusual for lines and dots with a width of just one pixel to be displayed.
FIG. 4 maps the luminance distribution in the horizontal direction of a white dot displayed as a single pixel in an LCD matrix. The red and blue luminance centroids R′ and B′ are considerably displaced from the center of the dot. Depending on the size of the pixel, the viewer may perceive a red tinge in the left part of the white dot and a blue tinge in the right part. The same tinged effect may also be visible in vertical white lines, and at the left and right edges of any white objects displayed against a darker background.
Another problem occurs when dark (for example, black) lines or letters are displayed on a bright (for example, white) background, to mimic the appearance of a printed page. It is generally true that bright objects tend to appear larger than dark objects. For example, a white pixel displayed against a black background appears larger than a black pixel displayed against a white background.
FIG. 5 shows the horizontal luminance distribution of a white pixel displayed on a black background. FIG. 6 shows the horizontal luminance distribution of a black pixel displayed on a white background. In both cases the display is a matrix-type display. ST0 to ST9 are pixels comprising respective sets of red, green, and blue cells. R0a to R9a are the luminance levels of the red cells, G0a to G9a are the luminance levels of the green cells, and B0a to B9a are the luminance levels of the blue cells.
The white pixel displayed as in FIG. 5 is perceived by the viewer as being larger than its actual size. Similarly, when fine bright lines are displayed on a dark background, they appear thicker than intended, and when bright text is displayed on a dark background, the letters may appear somewhat thickened. Still, the bright lines can be seen and the bright text can be read.
The black pixel displayed in FIG. 6, however, is perceived as being smaller than its actual size. When fine dark lines formed from dark dots are displayed on a bright background, the lines may become too faint to be seen easily. When dark text is displayed in a small font on a bright background, the letters may become difficult to read. These problems are aggravated in recent personal-computer display devices in which the resolution is increased and the pixel size is correspondingly reduced in order to increase the amount of information that can be displayed on the screen.
A known means of solving these problems is to use smoothing filters to reduce the sharpness of black-white boundaries, so that dark lines and letters do not appear too thin. Referring to FIG. 7, a conventional image display device in which this solution is adopted comprises analog-to-digital converters (ADCs) 1, 2, 3, smoothing units 5, 6, 7, and a display unit 8. The device receives analog input signals SR1, SG1, SB1 representing the red, green, and blue components of the image to be displayed. The analog-to-digital converters 1, 2, 3 convert these signals to corresponding digital signals SR2, SG2, SB2. These signals are filtered by the smoothing units 5, 6, 7 to obtain image data SR3, SG3, SB3 that are supplied to the display unit 8.
The smoothing units 5, 6, 7 operate with the characteristics FR1, FG1, FB1 illustrated in FIG. 8. These characteristics show how the image data SR2, SG2, SB2 for, in this case, three adjacent pixels STn, STn+1, STn+2 are used to calculate the filtered values for the central pixel STn+1, n being an arbitrary non-negative integer. The filtered luminance level SR3 of the red cell Rn+1 includes a large contribution from the original SR2 luminance level of this cell Rn+1 and smaller contributions from the original SR2 luminance levels of the adjacent red cells Rn and Rn+2, these two smaller contributions being mutually equal. Similarly, the filtered luminance level SG3 of green cell Gn+1 includes a large contribution from the SG2 level of cell Gn+1 and smaller, equal contributions from the SG2 levels of the adjacent green cells Gn and Gn+2. Likewise, the filtered luminance level SB3 of blue cell Bn+1 includes a large contribution from the SB2 level of cell Bn+1 and smaller, equal contributions from the SB2 levels of the adjacent blue cells Bn and Bn+2.
FIG. 9 shows the horizontal luminance distribution of a white pixel displayed on a black background after this filtering process. FIG. 10 shows the horizontal luminance distribution of a black pixel displayed on a white background after the same filtering process. These drawings may be compared with FIGS. 5 and 6. ST0 to ST9 are again pixels comprising respective sets of cells. R0b to R9b are the filtered luminance levels of the red cells, G0b to G9b are the filtered luminance levels of the green cells, and B0b to B9b are the filtered luminance levels of the blue cells.
In FIG. 9, the cell outputs in pixel ST2 are reduced by amounts R2c, G2c, B2c and the cell outputs in adjacent pixels ST1, ST3 are increased by amounts R1c, G1c, B1c, R3c, G3c, B3c, as compared with FIG. 5. In FIG. 10, the cell outputs in pixel ST7 are increased by double amounts R7c1+R7c2, G7c1+G7c2, B7c1+B7c2 and the cell outputs in adjacent pixels ST1, ST3 are reduced by amounts R6c, G6c, B6c, R8c, G8c, B8c, as compared with FIG. 6.
While this filtering process prevents the apparent decrease in size of dark dots and lines on bright backgrounds, it also leads to a certain loss of sharpness. In FIG. 9 the white dot in pixel ST2, which has an intrinsic tendency to appear larger than its actual size, is further enlarged by the redistribution of part of its luminance to adjacent pixels ST1 and ST3. In FIG. 10, the double increase in the luminance level of pixel ST7 implies a doubled loss of contrast with the background.
The conventional smoothing units 5, 6, 7 also fail to solve the problem of unwanted tinges of color at the right and left edges of white areas. FIG. 11 shows the locations of the red, green, and blue luminance centroids R′, G′, B′ of a one-pixel white dot after the conventional filtering process described above. Since the three primary colors are filtered with identical characteristics, the luminance centroids are separated just as much as they were in FIG. 4.
A further problem occurs when the input analog signals are transmitted to the image display device through cables with imperfect impedance matching, leading to ringing phenomena. FIG. 12 illustrates the ringing effect in the display of a single white dot of arbitrary width, the horizontal axis indicating horizontal position on the display screen, the vertical axis indicating luminance. The display screen is generally scanned from left to right, so ringing occurs at the right edge of the white dot. FIG. 13 illustrates the effect of the filtering process described above. The ringing is reduced at the right edge E1, but the left edge E2 is needlessly smoothed, reducing the sharpness of the displayed image.
The problems described above are not restricted to flat panel matrix-type displays, but can also be seen on CRT displays.